Since the hoisting performance of a main boom of a crane may be improved significantly by a single-cylinder pin-plug-in telescopic system, the system is widely used in large/medium tonnage crane products.
One end of a telescopic oil cylinder piston rod of the single-cylinder pin-plug-in system is fixed on the main boom, and a telescopic oil cylinder barrel slides in the sliding groove within each boom. Connections and separations between the telescopic oil cylinder and booms can be achieved through different combinations of a boom pin and a cylinder pin on the telescopic oil cylinder, and then stretching and contracting with a boom and stretching and contracting with the cylinder being idle can be achieved.
FIG. 1 is an exemplary schematic diagram of a dual-boom single-cylinder pin-plug-in telescopic system, wherein a telescopic oil cylinder can drive, through a cylinder pin, a secondary boom to stretch and contract, and a boom pin is used for connecting the secondary boom rigidly with the main boom. In practical application, the combination mostly includes five or more booms.
With a plurality of states such as stretching with a boom, contracting with a boom, stretching with the cylinder being idle and contracting with the cylinder being idle, different variable amplitude angles, and the number and combined operating conditions of telescopic booms, the load of the corresponding telescopic oil cylinder is not the same, and thus the value of oil pressure in the telescopic oil cylinder is not the same. The larger the opening of the solenoid valve on the oil way pipeline, the larger the flow and the swifter the stretching and contracting motion, so that regulating the opening size of the solenoid valve is a demanding task in order to ensure the smoothness of the stretching and contracting motion.
When oil leakage caused by damage of the oil way pipeline and damage of the valve circuit happens, the pressure in the telescopic oil cylinder cannot build up, and thus the value of the oil pressure will be relatively small. For example, as shown in FIG. 1, after the secondary boom stretches out, if the hydraulic oil in the large cavity is leaked off completely due to trouble in the large-cavity oil way, then after the boom pin is pulled off, there is no pressure support in the large cavity, so subjected to the gravity of the cylinder and the telescopic boom itself and under the huge pressure action of the small-cavity oil pressure, the stretching boom will fall rapidly, and it is very easy to cause the damage of the vehicle and a safety accident.
When the resistance to the stretching and contracting motion of the telescopic oil cylinder becomes greater due to the deformation of the main boom under force, lack of lubrication and maintenance for a long time, etc., so that the main boom cannot normally stretch and contract, it is easy to cause the pressure in the cylinder to be too high, and forced pressurization will cause damage to the whole system.
When it is detected that the end boom fails, the telescopic oil cylinder stretches excessively and the head of the telescopic oil cylinder collides with the head of the main boom, affected by the instant impact, the fluctuation in oil pressure will be very violent.
In the case of contracting with a boom, the pressure of the large cavity should be a bit lower than that of the small cavity; if the pressure difference is too large, the speed of contracting will be too fast; if the pressure difference is too small, the motion will be too slow. The system smoothness performance may be improved by real-time regulation of the solenoid valve and oil pump according to the pressure difference value.
In the prior art, the overflow valve technique is often used to prevent the oil pressure from being too high: an overflow valve is added to the hydraulic oil way pipeline, and when the oil pressure reaches the upper limit value of the overflow valve, the hydraulic oil flows back to the oil tank through the overflow valve, so as to ensure that the pressure in the oil way is not higher than a certain upper limit value, and then protect the system safety. However, the overflow valve technique can only ensure that the oil way pressure is not higher than a certain upper limit value, but the change in oil pressure cannot be known clearly. When the oil pressure is too low, the information about the oil pressure cannot be obtained, and at the same time the pump, solenoid valve and engine, etc. cannot be regulated or otherwise treated accordingly.
A boom position detection technique is also used in the prior art. According to that technique, the position of each bloom is detected by a proximity switch, and the boom position information, namely which boom's range the telescopic oil cylinder is within, is determined. When the end boom is detected, a corresponding judgment is made, so as to prevent overstretching of the cylinder. However, according to the boom position detection technique, the preventive effect can be exerted only on the overstretching, but when the boom stretching speed is too fast so that the cylinder and the head of the main boom collide, no corresponding treatment is given.
In addition, also used in the prior art is a telescopic oil cylinder length measuring technique: the stretching/contracting length of the single-cylinder pin-plug-in telescopic oil cylinder is measured by a boom position length sensor. However, that technology is only for detection of the results, and the reason why the stretching speed becomes faster or slower, or why incapability of stretching occurs cannot be determined.
In the prior art, mainly by operating a control handle, an operator controls the size of the solenoid valve opening and/or the pump displacement, and then controls the speed of the stretching and contracting motion. For example, the more the control handle is turned, the larger the solenoid valve opening and the flow are, the swifter the stretching and contracting motion will be. However, this way is based on the operator's operation on the control handle, so that a high requirement on the operator's operational skill is imposed. In addition, there is no quantified feedback information on the controlled variable of the controlled object (stretching/contracting speed of the telescopic oil cylinder). Thus, it is difficult to ensure the smoothness and safety of the system.
Actually, it is often difficult to pull the boom pin in the single-cylinder pin plug-in system. There are two main reasons: 1) the oil pressure in the telescopic oil cylinder cannot build up, the boom cannot stretch out, and thus the boom pin cannot be unhooked; and 2) the boom pin cylinder fails, so the boom pin cannot be pulled off. However, based on the prior art, the reason why the boom pin cannot be pulled out still cannot be determined.
All of the above examples have actually occurred, so it is necessary to detect the state of the telescopic oil cylinder.